1. Field of the Invention
The present invention relates to a terminal structure for a motor-driven pump and, more particularly, to a hermetic terminal structure suitable for use with a motor-driven pump such as a removable type canned motor pump.
2. Description of the Prior Art
The pump for sucking and discharging combustible and dangerous liquids, such as liquefied gas, liquefied ammonia or the like, is of a removable type; i.e., the pump itself is disposed in a tank containing the liquid in order to prevent a leakage of the liquid and is capable of pumping the liquid through a column pipe.
The removable type pump is normally used with a submerged motor. In the case where the liquid to be pumped is of the nature that corrodes copper, the motor used to drive the pump is canned so as to protect copper wires against the corrosive liquid. The pump and the motor are united into a unitary structure which is normally called "canned motor pump".
The canned motor pump has a terminal which interconnects a motor-side conductor cable extending from the motor of the motor pump unit and a power source-side conductor cable extending from an electric power supply source disposed outside a liquid storage tank. The terminal is usually installed on a casing of the canned motor pump. Because the canned motor pump is submerged in the liquid, the terminal is also submerged in the liquid.
The terminal is therefore required to be of a gas-tight structure which seals the interior of the terminal against the corrosive liquid such as liquefied ammonia. For this purpose, gaskets are conventionally employed in the terminal. In addition, the terminal element of the power source-side conductor cable, which is disposed in the terminal structure, is made of aluminium or stainless steel which is not readily corroded by the liquefied ammonia or the like.
A critical part of the conventional terminal structure which utilizes gaskets will be described with reference to FIG. 7 of the drawings.
A terminal 50 shown in FIG. 7 has terminal elements 52 and 54 respectively secured to an end of a power source-side conductor cable 51 and to an end of a motor-side conductor cable 53. The terminal 50 also has a terminal pin 55 of copper having opposite ends connected to the terminal elements 52 and 54 by bolts or screws. The terminal pin 55 extends through a through-hole in a terminal plate 56 of a stainless steel with an electrically insulating sleeve 57 interposed therebetween and with gaskets 63a and 63b interposed between the terminal pin 55 and the sleeve 57 and between the sleeve 57 and the terminal plate 56, respectively.
An upper pipe 58 has a flange 58a disposed in abutment engagement with the upper surface of the terminal plate 56. A lower pipe 59 has a flange 59a which is urged against the terminal plate 56 with a gasket 60 interposed therebetween. The two flanges 58a and 59a are clamped by bolts 61 (only one of which is shown) to sealingly encase the terminal pin 55.
The inner spaces in the upper and lower pipes 58 and 59 are filled with a filling material such as epoxy resin which seals the terminal elements 52 and 54.
However, the gaskets 63a and 63b used in the terminal 50 fall short of providing a satisfactory seal. The corrosive liquid such as liquefied ammonia discharged under a high pressure from the canned motor pump passes through a fine gap between, for example, the terminal plate 56 and the flange 58a and then through gaps between the insulating member 57, the gaskets 63a and 63b and other members until the high pressure corrosive liquid reaches the terminal pin 55 to corrode the same. If the terminal 50 is left un-repaired, the liquefied ammonia enters the motor section of the canned motor pump, with a result that the motor section is damaged due to corrosion by the high pressure liquefied ammonia.
On the other hand, Japanese Utility Model Publication No. 1-9096 published Mar. 13, 1989 discloses a gas-tight terminal structure for a canned motor pump for use in a tank containing a liquid, such as liquefied natural gas or liquefied propane gas, which, unlike liquefied ammonia, does not cause corrosion of copper.
The gas-tight terminal structure disclosed in the Japanese publication referred to above has a terminal base provided on a casing of a motor pump unit and having formed therein a through-hole through which extends a ceramic terminal element which supports a copper conductor extending therethrough. Metal rings are interposed respectively between the ceramic terminal element and the terminal base and between the ceramic terminal element and the copper conductor and are secured respectively to the pair of the ceramic terminal element and the terminal base and to the pair of ceramic terminal element and the copper conductor by soldering or welding. The terminal structure of the above-described gas-tight structure is disposed in a housing mounted on the casing of the canned motor pump.
The gas-tight terminal structure disclosed in the Japanese publication referred to above can provide a extremely high gas-tightness because the pair of ceramic terminal element and the conductor and the pair of ceramic terminal element and the terminal base are sealingly connected together by welding or soldering.
However, the gas-tight terminal structure disclosed in the Japanese publication referred to above cannot be directly applied to a canned motor pump which is operated to deal with a corrosive liquid such as liquefied ammonia which shows a high corrosive nature relative to copper.
This is because, while the gas-tight terminal structure itself has a high gas-tightness, the copper conductor is corroded by the corrosive liquid which invades into the housing which accommodates the gas-tight terminal structure, so that the gas-tightness of the terminal structure damaged.
However, the conductor in the gas-tight terminal structure cannot be easily replaced by a metal such as aluminium which is corrosion-resistant to liquefied ammonia because no technique has ever been developed for easily securing a metal ring to a conductor metal such as aluminium by soldering or welding.
Accordingly, the gas-tight terminal structure disclosed in the Japanese publication referred to above cannot be applied to a terminal box structure for a canned motor pump to be submerged in a corrosive liquid such as liquefied ammonia, with a result that there is no way other than relying on gaskets so as to achieve the required gas-tightness of the terminal structure. However, the use of gaskets in the terminal structure can provide a gas-tightness to only a limited extent, as pointed out above, which disadvantageously induces an occurrence of damage in the pump.